We are developing a new method for immunoassay based on long time autocorrelation of fluorescent intensity fluctuations. The method is designed to distinguish between fluorescence tag molecules which are free in solution (or attached to small macromolecules) as opposed to fluorescent tags which are attached to micron-sized carrier particles which diffuse very slowly. This method, in principle, is able to discriminate such slowly diffusing sources of fluorescent light from large contaminating backgrounds. The goal is to produce an assay which is simple, inexpensive and as sensitive as the competing method of radioimmunoassay (RIA) suitable for small scale clinical applications.